[PATCH tip/core/rcu 39/88] srcu: Make SRCU be once again optional

[Paul E. McKenney]

Commit d160a727c40e ("srcu: Make SRCU be built by default") in response
to build errors, which were caused by code that included srcu.h
despite !SRCU.  However, srcutiny.o is almost 2K of code, which is not
insignificant for those attempting to run the Linux kernel on IoT devices.
This commit therefore makes SRCU be once again optional, and adjusts
srcu.h to allow error-free inclusion in !SRCU kernel builds.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Acked-by: Nicolas Pitre <nico@xxxxxxxxxx>
---
 include/linux/srcu.h | 7 ++++++-
 init/Kconfig         | 1 -
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/include/linux/srcu.h b/include/linux/srcu.h
index 167ad8831aaf..3a684db12213 100644
--- a/include/linux/srcu.h
+++ b/include/linux/srcu.h
@@ -62,8 +62,13 @@ int init_srcu_struct(struct srcu_struct *sp);
 #include <linux/srcutree.h>
 #elif defined(CONFIG_CLASSIC_SRCU)
 #include <linux/srcuclassic.h>
-#else
+#elif defined(CONFIG_SRCU)
 #error "Unknown SRCU implementation specified to kernel configuration"
+#else
+
+/* Dummy definition for things like notifiers.  Actual use gets link error. */
+struct srcu_struct { };
+
 #endif
 
 /**
diff --git a/init/Kconfig b/init/Kconfig
index 1d3475fc9496..d928a3724af9 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -521,7 +521,6 @@ config RCU_EXPERT
 
 config SRCU
 	bool
-	default y
 	help
 	  This option selects the sleepable version of RCU. This version
 	  permits arbitrary sleeping or blocking within RCU read-side critical
-- 
2.5.2